OPERATING SYSTEMS. Prescribed Text Book Operating System Principles, Seventh Edition By Abraham Silberschatz, Peter Baer Galvin and Greg Gagne

Transcription

1 OPERATING SYSTEMS Prescribed Text Book Operating System Principles, Seventh Edition By Abraham Silberschatz, Peter Baer Galvin and Greg Gagne

2 OVERVIEW An operating system is a program that manages the computer hardware. It provided a basis for application programs and acts as an intermediary between the computer user and the computer hardware. Main frame operating systems are designed primarily to optimize utilization of hardware. PC operating systems support complex games, business applications etc. Operating systems for handheld computers are designed to provide an environment in which a user can easily interface with the computer to execute programs. What operating systems do: A computer system can be divided roughly into four components the hardware, the operating system, the application programs and the users. The hardware which consists of CPU, memory and I/O devices, provides the basic computing resources for the system. The application programs define the ways in which these resources are used to solve users computing problems. The operating system controls and co-ordinates the use of hardware among the various application programs for the various users.

3 Operating system from the user view- The user s view of the computer varies according to the interface being used. While designing a PC for one user, the goal is to maximize the work that the user is performing. Here OS is designed mostly for ease of use. In another case the user sits at a terminal connected to a main frame or minicomputer. Other users can access the same computer through other terminals. OS here is designed to maximize resource utilization to assure that all available CPU time, memory and I/O are used efficiently. In other cases, users sit at workstations connected to networks of other workstations and servers. These users have dedicated resources but they also share resources such as networking and servers. Here OS is designed to compromise between individual usability and resource utilization. Operating system from the system view- From the computer s point of view, OS is the program which is widely involved with hardware. Hence OS can be viewed as resource allocator where in resources are CPU time, memory space, file storage space, I/O devices etc. OS must decide how to allocate these resources to specific programs and users so that it can operate the computer system efficiently. OS is also a control program. A control program manages the execution of user programs to prevent errors and improper use of computer. It is concerned with the operation and control of I/O devices. Defining operating systems- OS exists because they offer a reasonable way to solve the problem of creating a usable computing system. Goal of computer systems is to execute user program and to make solving user problems easier. Hence hardware is constructed. Since hardware alone is not easy to use, application programs are developed. Operating System Structure- An OS provides an environment within which programs are executed. One of the most important aspects of OS is its ability to multi program. Multi programming increases CPU utilization by organizing jobs (code and data) so that the CPU always has one to execute. OS keeps several jobs in memory. This set of jobs can be a subset of jobs kept in the job pool which contains all jobs that enter the system. OS picks and begins to execute one of the jobs in

4 memory. The job may have to wait for some task, such as I/O operation to complete. In a non multi programmed system, OS simply switches to and executes another job. When that job needs to wait, CPU is switched to another job and so on. As long as at least on job needs to execute, CPU is never idle. Multi programmed systems provide an environment in which the various system resources are utilized effectively but they do not provide for user interaction with the computer system. Time sharing or multi tasking is a logical extension of multi programming. In time sharing systems, CPU executes multiple jobs by switching among them but the switches occur so frequently that the users can interact with each program while it is running. Time sharing requires an interactive computer system which provides direct communication between the user and the system. A time shared operating system allows many users to share the computer simultaneously. It uses CPU scheduling and multi programming to provide each user with a small portion of a time shared computer. A program loaded into memory and executing is called a process. Time sharing and multi programming require several jobs to be kept simultaneously in memory. Since main memory is too small to accommodate all jobs, the jobs are kept initially on the disk in the job pool. This pool consists of all processes residing on disk awaiting allocation of main memory. If several jobs are ready to be brought into memory and there is not enough space, then the system must choose among them. Making this decision is job scheduling. Having several programs in memory at the same time requires some form of memory management. If several jobs are ready to run at the same time, the system must choose among them. Making this decision is CPU scheduling.

5 In a time sharing system, the operating system must ensure reasonable response time which is accomplished through swapping where processes are swapped in and out of main memory to the disk. Virtual memory is a technique that allows the execution of a process that is not completely in memory. It enables users to run programs that are larger than actual physical memory. Protection and security- If a computer system has multiple users and allows the concurrent execution of multiple processes, then access to data must be regulated. Hence mechanisms ensure that files, memory segments, CPU and other resources can be operated on by only those processes that have gained proper authorization from the OS. Protection is a mechanism for controlling the access of processes or users to the resources defined by a computer system. This mechanism must provide means for specification of the controls to be imposed and means for enforcement. Protection improves reliability by detecting latent errors at the interfaces between component sub systems. It is the job of security to defend a system from external and internal attacks. Such attacks spread across a huge range and include viruses and worms, denial of service attacks, identity theft and theft of service. Distributed Systems- A distributed system is a collection of physically separate, possibly heterogeneous computer systems that are networked to provide the users with access to the various resources that the system maintains. Access to a shared resource increases computation speed, functionality, data availability and reliability. The protocols that create a distributed system can greatly affect that system s utility and popularity. A network is a communication path between two or more systems. Distributed systems depend on networking for their functionality. Networks are characterized based on the distances between their nodes. A local area network (LAN) connects computers within a room, a floor or a building. A wide area network (WAN) links buildings, cities or countries. A metropolitan area network (MAN) could link buildings within a city. Special Purpose Systems

6 Classes of computers whose functions are limited and objective is to deal with limited computation domains- Real Time Embedded Systems: Embedded computers are devices found from car engines and manufacturing robots to VCR s and microwave ovens. These have specific tasks to accomplish. Embedded systems almost always run real time operating system. Multimedia systems: Most operating systems are designed to handle conventional data such as text files, programs, and word processing documents and spread sheets. A recent trend is incorporation of multimedia data into computer systems. Multimedia data consist of audio and video files as well as conventional files. Handheld systems: Handheld systems include personal digital assistants (PDA s), cellular telephones many of which use special purpose embedded operating systems.

7 OS services- SYSTEM STRUCTURES OS provides an environment for execution of programs. It provides certain services to programs and to the users of those programs. OS services are provided for the convenience of the programmer, to make the programming task easier. One set of SOS services provides functions that are helpful to the user a. User interface: All OS have a user interface(ui).interfaces are of three types- Command Line Interface: uses text commands and a method for entering them Batch interface: commands and directives to control those commands are entered into files and those files are executed. Graphical user interface: This is a window system with a pointing device to direct I/O, choose from menus and make selections and a keyboard to enter text. b. Program execution: System must be able to load a program into memory and run that program. The program must be able to end its execution either normally or abnormally. c. I/O operations: A running program may require I/O which may involve a file or an I/O device. For efficiency and protection, users cannot control I/O devices directly. d. File system manipulation: Programs need to read and write files and directories. They also need to create and delete them by name, search for a given file, and list file information. e. Communications: One process might need to exchange information with another process. Such communication may occur between processes that are executing on the same computer or between processes that are executing on different computer systems tied together by a computer network. Communications may be implemented via shared memory or through message passing. f. Error detection: OS needs to be constantly aware of possible errors. Errors may occur in the CPU and memory hardware, in I/O devices and in the user program. For each type of error, OS takes appropriate action to ensure correct and consistent computing. Another set of OS functions exist for ensuring efficient operation of the system. They are- a. Resource allocation: When there are multiple users or multiple jobs running at the same time, resources must be allocated to each of them. Different types of resources such as CPU cycles, main memory and file storage are managed by the operating system.

8 b. Accounting: Keeping track of which users use how much and what kinds of computer resources. c. Protection and security: Controlling the use of information stored in a multiuser or networked computer system. Protection involves ensuring that all access to system resources is controlled. Security starts with requiring each user to authenticate himself or herself to the system by means of password and to gain access to system resources. System Calls System calls provide an interface to the services made available by an operating system. An example to illustrate how system calls are used: Writing a simple program to read data from one file and copy them to another file- a) First input required is names of two files input file and output file. Names can be specified in many ways- One approach is for the program to ask the user for the names of two files. In an interactive system, this approach will require a sequence of system calls, to write a prompting message on screen and then read from the keyboard the characters that define the two files. On mouse based and icon based systems, a menu of file names is displayed in a window where the user can use the mouse to select the source names and a window can be opened for the destination name to be specified. b) Once the two file names are obtained, program must open the input file and create the output file. Each of these operations requires another system call. Possible error conditions

9 When the program tries to open input file, no file of that name may exist or file is protected against access. Program prints a message on console and terminates abnormally. If input file exists, we must create a new output file. If the output file with the same name exists, the situation caused the program to abort or delete the existing file and create a new one. Another option is to ask the user(via a sequence of system calls) whether to replace the existing file or to abort the program. When both files are set up, a loop reads from the input file and writes to the output file (system calls respectively). Each read and write must return status information regarding various possible error conditions. After entire file is copied, program closes both files, write a message to the console or window and finally terminate normally. Application developers design programs according to application programming interface (API). API specifies set of functions that are available to an application programmer. Three of the most common API s available to application programmers are the Win32API for Windows Systems; POSIX API for POSIX based systems (which include all versions of UNIX, Linux and Mac OS X) and Java API for designing programs that run on Java virtual machine. Pictorial representation of system calls- The functions that make up the API typically invoke the actual system calls on behalf of the application programmer. Benefits of programming rather than invoking actual system calls:

10 Program portability An application programmer designing a program using an API can expect program to compile and run on any system that supports the same API. Actual system calls can be more detailed and difficult to work with than the API available to an application programmer. The run time support system ( a set of functions built into libraries included with a compiler) for most programming languages provides a system call interface that serves as a link to system calls made available by OS. The system call interface intercepts function calls in the API and invokes the necessary system call within the operating system. A number is associated with each system call and the system call interface maintains a table indexed according to these numbers. System call interface then invokes the intended system call in the OS kernel and returns the status of the system call and return any values. System calls occur in different ways, depending on the computer in use more information is required than simply the identity of the desired system call. The exact type and amount of information vary according to the particular OS and call. Three general methods are used to pass parameters to OS- I. Pass the parameters in registers II. Storing parameters in blocks or tables in memory and the address of the block id passed as a parameter in a register III. Placing or pushing parameters onto the stack by the program and popping off the stack by the OS. Types of system calls Five major categories: 1) Process control o end, abort

11 o load, execute o create process, terminate process o get process attributes, set process attributes o wait for time o wait event, signal event o allocate and free memory 2) File Management o create file, delete file o open, close o read, write, reposition o get file attributes, set file attributes 3) Device management o request device, release device o read, write, reposition o get device attributes, set device attributes o logically attach or detach devices 4) Information maintenance o get time or date, set time or date o get system data, set system data o get process, file or device attributes o set process, file or device attributes 5) Communications o create, delete communication connection o send, receive messages o transfer status information o attach or detach remote devices System Programs

12 System programs provide a convenient environment for program development and execution. They can be divided into these categories- File management: These programs create, delete, copy, rename, print, dump, list and manipulate files and directories. Status information: Some programs ask the system for the date, time, and amount of available memory or disk space, number of users. File modification: Text editors may be available to create and modify the content of files stored on disk or other storage devices. Programming language support: Compilers, assemblers, debuggers and interpreters for common programming languages are often provided to the user with the OS. Program loading and execution: Once a program is assembled or compiled, it must be loaded into memory to be executed. System provides absolute loaders, relocatable loaders, linkage editors and overlay loaders. Communications: These programs provide the mechanism for creating virtual connections among processes, users and computer systems. In addition to systems programs, OS are supplied with programs that are useful in solving common problems or performing operations. Such programs include web browsers, word processors and text formatters, spread sheets, database systems etc. These programs are known as system utilities or application programs. Operating System Structure Simple structure:

13 Operating systems of commercial systems started as a small, simple and limited systems. Example is MS-DOS. It was written to provide the most functionality in the least space, so it was not divided into modules. But the interfaces and levels of functionality are not separated. It was also limited by the hardware. Layered approach: With proper hardware support, OS can be broken into pieces that are smaller and more appropriate. OS can then retain much greater control over the computer and over the applications that make use of the computer. Under the top down approach, the overall functionality and features are determined and are separated into components. A system can be made modular in many ways one method is the layered approach in which the OS is broken up into number of layers (levels). The bottom layer is the hardware and the highest layer is the user interface.

14 The main advantage of layered approach is simplicity of construction and debugging. The layers are selected so that each uses functions and services of only lower level layers. This approach simplifies debugging and system verification. The major difficulty with layered approach involves defining the various layers. They tend to be less efficient than other types. Micro kernels: This method structures the OS by removing all non essential components from the kernel and implementing them as system and user level programs which results in a smaller kernel. Micro kernels provide minimal process and memory management in addition to a communication facility. The main function of micro kernel is to provide a communication facility between the client program and the various services that are also running in user space. Communication is provided by message passing. Advantage of the micro kernel approach is ease of extending the operating system. All new users are added to user space and hence do not require modification of the kernel. The resulting operating system is easier to port from one hardware design to another. Microkernel also provides more security and reliability since most services are running as user processes. But micro kernels can suffer from performance decreases due to increased system function over head. Modules:

15 The best current methodology for operating system design involves using object oriented programming techniques to create a modular kernel. The kernel has a set of core components and dynamically links in additional services either during boot time or run time. Ex: Solaris loadable kernel modules Solaris OS structure is organized around a core kernel with seven types of loadable kernels. Such a design allows the kernel to provide core services and also allows certain features to be implemented dynamically. Operating System Generation Operating systems are designed to run on any class of machines at a variety of sites with a variety of peripheral configurations. The system must then be configured or generated for

16 each specific computer site, a process known as system generation (SYSGEN). This SYSGEN program reads from a given file or asks the operator of the system for information concerning the specific configuration of the hardware system or probes the hardware directly to determine what components are there. The following information must be determined: a) What CPU is to be used? What options are installed? For multiple CPU systems, each CPU system must be described. b) How much memory is available? c) What devices are available? d) What operating system options are desired or what parameter values are to be used? Once this information is determined, it can be used in several ways. It can be used by the system administrator to modify a copy of the source code of the OS. OS is then completely compiled. It is also possible to construct a system that is completely table driven. All the code is always part of the system and selection occurs at execution time rather than compile time or link time. The major differences among these approaches are the size and generality of the generated system and the ease of modification as the hardware configuration changes.